Makes haproxy nrpe check thresholds configurable

[Benjamin Allot]

The nrpe check_haproxy_queue_depth has non configurable values.

CURRQthrsh=0
MAXQthrsh=100

If this MAXQthrsh is reached, the alert won't go away once it goes down, and this is not configurable.

I suggest we address both points by:
* making the threshold configurable
* either an opt-in or a "sane default" to clear the alert when the queue is going under the threshold